Mechanisms of Genome Instability in the Fragile X-Related Disorders

• Published in: Genes Vol. 12; no. 10; p. 1633
• Main Authors: Hayward, Bruce E, Usdin, Karen
• Format: Journal Article
• Language: English
• Published: Switzerland MDPI AG 17.10.2021; MDPI
• Subjects: 5' Untranslated Regions; Alleles; Aneuploidy; Ataxia; Ataxia - genetics; Ataxia - physiopathology; Cell cycle; Cell division; DNA Repeat Expansion - genetics; DNA Replication - genetics; Female; FMR1 gene; FMR1 protein; Folic acid; Fragile X Mental Retardation Protein - genetics; Fragile X syndrome; Fragile X Syndrome - genetics; Fragile X Syndrome - physiopathology; Genomes; Genomic instability; Genomic Instability - genetics; Heredity; Humans; Intellectual disabilities; Males; Mutation; Ovaries; Primary Ovarian Insufficiency - genetics; Primary Ovarian Insufficiency - physiopathology; Proteins; Review; Tremor; Tremor - genetics; Tremor - physiopathology; Trinucleotide Repeat Expansion - genetics; Vitamin B; X chromosomes; base excision repair (BER); repeat contractions; repeat expansion; break induced replication (BIR); aneuploidy; repeat mosaicism; mitotic DNA synthesis (MiDAS); chromosome fragility; microhomology mediated end-joining (MMEJ)
• ISSN: 2073-4425; 2073-4425
• DOI: 10.3390/genes12101633

The Fragile X-related disorders (FXDs), which include the intellectual disability fragile X syndrome (FXS), are disorders caused by expansion of a CGG-repeat tract in the 5' UTR of the X-linked gene. These disorders are named for FRAXA, the folate-sensitive fragile site that localizes with the CGG-repeat in individuals with FXS. Two pathological allele size classes are distinguished. Premutation (PM) alleles have 54-200 repeats and confer the risk of fragile X-associated tremor/ataxia syndrome (FXTAS) and fragile X-associated primary ovarian insufficiency (FXPOI). PM alleles are prone to both somatic and germline expansion, with female PM carriers being at risk of having a child with >200+ repeats. Inheritance of such full mutation (FM) alleles causes FXS. Contractions of PM and FM alleles can also occur. As a result, many carriers are mosaic for different sized alleles, with the clinical presentation depending on the proportions of these alleles in affected tissues. Furthermore, it has become apparent that the chromosomal fragility of FXS individuals reflects an underlying problem that can lead to chromosomal numerical and structural abnormalities. Thus, large numbers of CGG-repeats in the gene predisposes individuals to multiple forms of genome instability. This review will discuss our current understanding of these processes.